The present invention relates to a video server for transmitting a video signal in parallel to a plurality of locations using the Internet protocol, and more particularly, to a video server for a video distribution system which enables transmission of a video signal through a transmission path, which cannot use IP multicast, through protocol conversion.
Simultaneous transmission of a signal such as a video signal to multiple locations, utilizing a protocol such as the Internet protocol, is referred to as “multicast,” and a number of methods for realizing the multicast have already been proposed, with some of such methods already brought into practical use. A representative one of these methods is the Internet multicast technology which has been published as a standard designated “RFC1112” of Internet Engineering Task Force (IETF).
A critical component of the multicast technology is a host group model. Multicast information is transmitted to a host group indicated by some multicast address, such that each of terminals attempting to share the multicast information receives the information addressed to the host group. In a network based on the Internet protocol, multicast information is accompanied by a “multicast address,” used as a destination address, which indicates that the information is multicast.
Upon receiving multicast information, a device for controlling an information transmission path, i.e., a “router” reads its multicast address, and transmits the multicast information to a path to which a terminal belonging to an associated host group is connected. In this event, when a plurality of terminals belong to the associated host group and they are located on different paths, the router copies the information intended for transmission, and transmits the copies to the respective paths. This scheme allows for a large reduction in the amount of transmitted information, as compared with independent transmission of information from an origination to all terminals belonging to a host group.
However, the effectiveness of the multicast-based transmission is limited only to those paths which support the multicast from an origination to all terminals belonging to a host group. Additionally, in the current Internet environment, firewalls are installed everywhere as required for the security, preventing the Internet protocol information from freely passing therethrough.
The firewall refers to a device for examining information which is going to pass therethrough to block information other than that regarded as safe. Multicast information is generally blocked by the firewall. Therefore, a special setting is required for multicast information to pass through the firewall. However, it is quite difficult to pass multicast information through in the current Internet in which firewalls are installed everywhere and managed independently by different organizations.
Therefore, for transmitting the same video simultaneously to a plurality of terminals when firewalls are interposed between a video server and the terminals, the video is conventionally transmitted individually from the video server to each of the terminals. In this event, the amount of information transmitted from the video server to the terminals is increased in proportion to the number of receiving terminals, resulting in a problem of a higher transmission cost.
On the other hand, many of protocols for passing information through a firewall are inherently intended for file transfer, so that although they can transmit information without errors, they do not guarantee when the information will arrive at a particular terminal. Applications which require the multicast often involve real-time transmission such as transmission of video and audio contents. Therefore, a mechanism for matching the transmission rate between reception and transmission of video and audio contents is required for protocols which pass the information through firewalls.